Computer printers based on a printing mechanism that expels droplets of ink toward the paper are often referred to as "inkjet" printers. These printers cost substantially less than laser based printers while providing equivalent resolutions and the ability to print in color. However, the cost of the ink cartridges raises the per page cost of black and white printing to above that obtainable with laser based printers.
Inkjet printers utilize a print head that has a number of nozzles through which the ink is propelled. In one type of printer head, the ink droplets are propelled by heating the ink in a capillary tube such that the expansion of the heated ink forces the ink nearest the end of the capillary tube to be expelled. Each nozzle has one such capillary tube and the related circuitry to drive the heating element. The circuitry is typically contained on a "chip" that is part of the print head. The cartridge is normally thrown out when the ink supply in one of the reservoirs is exhausted; however, kits for refilling the ink reservoirs are available. Unfortunately, the print heads have a finite lifetime determined by wear and clogging of the nozzles. Hence, relatively few refillings may be utilized before the quality of the printing becomes unacceptable.
InkJet print heads do not always shoot straight to the predicted print locations. The location at which the drop lands and the shape of the drop are partially determined by the driving voltages used to expel the droplet. The speed with which the droplet is expelled can be controlled by the power applied to the heater that expands the liquid behind the droplet. Since the print head is also moving during the printing process, the droplet lands at a location that depends on the speed of the droplet and the print head speed. In addition, the shape of the spot on the paper is also partially determined by the speed with which the droplet is expelled. If the droplet is expelled at too high a velocity, the droplet will breakup in flight or splatter when it hits the page.
After manufacture, the print heads are tested to eliminate those that shoot with less than the required precision. In addition, normal wear on the print head changes the shape and the trajectory of the ink drops so that ragged lines with uncontrolled spaces may appear affecting the quality of the display. The need to throw out print heads that do not shoot within limits after manufacture lowers the yield of the production line, and hence, increases the cost of the print heads. The wear-related failures shorten the life of the print heads, and hence, also increase the cost of printing with inkjet printers.
If the results of each nozzle could be sensed during a calibration sequence, many of the problems that cause the head to shoot poorly could be corrected by adjusting the power delivered to the nozzle and the nozzle firing timing to compensate for the problems. For example, a nozzle that is delivering a droplet that is splattering could be corrected by reducing the power used to expel the droplets, and thereby, reduce the impact speed of the droplet on the paper. Similarly, the position of the dot on the paper along the direction of motion of the print head can be altered by adjusting the timing of the nozzle firing.
In addition, periodic calibration would enable corrections to be made over the life of the cartridge thereby increasing the usefull lifetime of the cartridge. As noted above, even when refilled, inkjet print heads have a relatively short lifetime because of wear. Some of the wear related problems can be corrected by adjusting the driving parameters of the individual nozzles. Hence, providing a calibration system on the printer can also extend the useful lifetime of the print head.
Broadly, it is the object of the present invention to provide an improved inkjet print head.
It is a further object of the present invention to provide an inkjet print head that can sense the location at which various nozzles deliver ink drops.
It is a still further object of the present invention to provide an inkjet print head that can sense the shape of the dots generated by each of the nozzles.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.